1. Field of the Invention
The present invention relates to a gas sensor having a gas sensor element for detecting the concentration of a specific gas, such as NOx, contained in a combustion gas or an exhaust gas from, for instance, a combustor or an internal combustion engine.
2. Description of the Related Art
As regulation of exhaust gas from an internal combustion engine, such as an automobile, is tightened, there has been a need to reduce the quantity of nitrogen oxides (NOx) in the exhaust gas. Further, an NOx sensor capable of directly measuring the concentration of NOx has been developed.
The NOx sensor has a gas sensor element. The gas sensor element has one or a plurality of cells, each of which comprises a pair of electrodes formed on a surface of a solid electrolytic layer exhibiting oxygen ion conductivity, such as zirconia. An oxygen concentration detection cell measures the concentration of oxygen in a first measurement chamber in mutual communication with a space for a gas to be measured. A first pumping cell controls (pumps in or pumps out) oxygen in the first measurement chamber such that a target oxygen concentration is achieved in the first measurement chamber. Moreover, a gas to be measured having a controlled oxygen concentration flows from the first measurement chamber to a second measurement chamber. A constant voltage is applied to a second pumping cell, whereby NOx contained in the gas to be measured in the second measurement chamber is decomposed into N2 and O2. At this time, a second pump current flowing between a pair of electrodes of the second pumping cell is measured, whereby the concentration of NOx in the gas to be measured is detected.
In such an NOx sensor, even when the concentration of NOx is zero, the second pump current comes to a predetermined current value (a so-called offset value). Hence, the concentration of NOx is converted by subtracting the offset value from the second pump current. Incidentally, there is a problem in that the offset value itself varies with a change in the temperature of the NOx sensor or the like, to thus deteriorate the measurement accuracy. The problem may be ascribed to a phenomenon of an increase in the temperature of the second pumping cell affecting conduction of electrons of a solid electrolytic layer.
Accordingly, a technique has been developed for making dense a pattern 94a of a heater for heating the sensor element section, which faces a leading-end side of a sensor element 100, and eliminating a pattern 94c from an area corresponding to a second pumping cell (on a rear end of the sensor element 100). In this manner the temperature of the second pumping cell is reduced, to thereby prevent the occurrence of variations in the offset value (see Patent Document 1).
[Patent Document 1] JP-A-10-318979 (a front page, FIG. 3)
3. Problems Solved by the Invention
However, in the technique described in Patent Document 1, the line width of the pattern section 94c removed from the heater is made equal to that of a lead section of the heater. Therefore, this area of the heater does not generate heat and is influenced by external temperatures. Accordingly, the temperature of that area may be changed by an external heat source (e.g., transfer of engine heat from a housing to which the sensor element section is attached), which in turn induces variations in offset value.